IFN-GAMMA-INDUCED MHC CLASS-II GENE-EXPRESSION IS SUPPRESSED IN ENDOTHELIAL-CELLS BY DEXTRAN SULFATE

IFN-gamma-activated endothelial cells actively participate in initiati ng immune responses by interacting with immunocompetent cells via clas s II MHC proteins. In this study, dextran sulfate, a synthetic heparin analogue, was shown to selectively inhibit IFN-gamma-induced surface expression of HLA-DR molecules by human umbilical cord vascular endoth elial cells, but not other cytokine-induced molecules such as ELAM-1 o r ICAM-1. Inhibition occurred regardless of whether dextran sulfate wa s added 24 h before, at the same time as, or 24 h after IFN-gamma stim ulation of cells. In addition, both high (500 kDa) and low (5 kDa) mol ecular mass dextran sulfate molecules were able to block class II expr ession, whereas treating cells with naturally occurring polysulfated g lycosaminoglycans such as heparin, heparan, and chondroitin sulfate di d not produce any suppressive effects. Radiolabeling of cells with [S- 35]methionine followed by radioimmunoprecipitation using anti-HLA-DR a lpha mAb demonstrated that biosynthesis of class II proteins was speci fically blocked. RT-PCR and Southern blotting were utilized to examine transcription of the HLA-DR alpha gene and demonstrated an absence of HLA-DR alpha mRNA from dextran sulfate-treated and IFN-gamma-induced cells. Dextran sulfate also prevented transcription of the gene encodi ng CIITA, a transactivator protein required for IFN-gamma-inducible ex pression of class II genes. Thus, dextran sulfate apparently inhibited this step or an earlier one in the intracellular signaling pathway fo r IFN-gamma in human endothelial cells, subsequent to IFN-gamma bindin g to its cell surface receptor.